A:up; dow
B:off; u
C:down; u
D:through; dow
It was the second time that we ________ in holding such important parties.
① 23:0.5=
②2.7:1.8=
③ 2.43.2=
④ 115 米:45分米=
⑤500厘米:5千米=
⑥1.2小时:40分=
(50+13)÷9=____ 90-9-60=____ 7×(26-20)=____
从物质循环的观点来看,人体中的碳元素的最终来源是( )